Looking toward next-generation mobile communication systems, various studies have been conducted on radio transmission schemes suitable for high-speed packet transmissions capable of achieving data rates over 100 Mbps. Broadband is necessary for the frequency band used for such high-speed transmission, and the use of a bandwidth to the extent of 100 MHz has been studied.
It is known that, when this kind of broadband transmission is performed in mobile communications, a communication channel becomes a frequency selective channel comprised of a plurality of paths having different delays. That is, in broadband transmission for mobile communications, ISI (InterSymbol Interference), which refers to the interference that a preceding symbol causes against the following symbol, and BER (Bit Error Rate) characteristics deteriorate. In addition, the frequency selective channel refers to a channel where the channel transfer function varies within a frequency bandwidth, and the spectrum of the received signal via such a channel is distorted.
A technique to remove the influence of ISI and improve BER characteristics relates to the equalization technology. Especially, MLSE (Maximum Likelihood Sequence Estimation) is known as one of time domain equalization techniques. However, in MLSE, when the number of paths increases, the configurations of equalizers become more complex, and the amount of calculation required for the equalization increases exponentially. Consequently, gaining attention recently directs toward FDE (Frequency Domain Equalization) (see, for example, Patent Document 1) as an equalization technique where the configuration of the equalizer does not rely on the number of paths.
In FDE, received signal blocks are broken into orthogonal frequency components by FFT (fast Fourier transform), and, after the orthogonal frequency components are multiplied by equalization weights approximating the reciprocal of the channel transfer function, the multiplied frequency components are converted into a time domain signal by IFFT (inverse fast Fourier transform). This FDE makes it possible to compensate for the distortion of the spectrum of the received signal, consequently reducing ISI and improving BER characteristics. In addition, as for equalization weights, MMSE (Minimum Mean Square Error) weights, which minimize the minimum mean square error between the frequency components after equalization and transmission signal components, provide optimal BER characteristics. Patent Document 1: “Frequency domain equalization for single-carrier broadband wireless systems”, D. Falconer, S. L. Ariyavistakul, A. Benyamin-Seeyar, and B. Eidson, IEEE Communications Magazine., vol. 40, pp. 58-66, April 2002.